Specific Aim 2) To determine whether age-related declines in AMPK signaling are involved in the reductions in fat oxidation and insulin resistance that occur in aging. Using an in vitro primary muscle cell culture system, we will test the hypotheses that (i) reduced AMPK signaling in myotubes from older subjects leads to decreased mitochondrial fatty acid oxidation; and (ii) reduced AMPK signaling and fat oxidation in myotubes from older subjects will result in increased susceptibility to fat-induced insulin resistance.